1. Field of the Invention
This invention relates to the field of apparatus for manufacturing film-type resistors wherein a film of resistive material is provided on the exterior surface of a cylinder. Throughout this specification and claims, the word "cylinder" is used in its conventional sense (a surface generated by a straight line moving parallel to a fixed straight axis and intersecting a fixed circle, the circle lying in a plane perpendicular to said axis, the circle having said axis at its center).
2. Description of Prior Art
It has long been thought that silk-screening is impractical in the field of film-type cylindrical resistors, particularly those having very small diameters and/or being of the economical and moisture-resistant solid ceramic variety. It is believed that the thinking of prior-art workers was largely locked to the traditional helical film pattern, and a helix may not be silk-screened without overprinting part of the pattern. Overprinting is often fatal to the integrity of a film-type resistor where the film is a line, since overprinting produces disuniform film thicknesses as well as tending toward smudging and thus short-circuiting of turns of the helix. It is emphasized that (a) overprinting is especially intolerable where the elongated strip of film (forming the pattern) is narrow, and (b) narrow strips are often highly desirable, for example to increase resistance. Overprinting is also especially intolerable where the narrow strips are caused to be close together, to minimize inductance and to mimimize the size of the resistor.
A very important consideration in the manufacture of film-type resistors is the degree of utilization of the available surface area. This is especially true in the small-diameter "toothpick" size resistors, it being noted that the circumference of such a resistor is often less than 200 thousandths of an inch. If the gap between the printed portions of such resistors were large, such as close to 100 thousandths of an inch, only 50 percent of the surface area would be available for film-coating.
Silk-screening is (as mentioned above) particularly difficult to perform relative to solid ceramic cylindrical substrates, which are economical to manufacture by extrusion and centerless-grinding techniques, and which have highly superior qualities relative to strength and moisture resistance. Being solid, as distinguished from tubular, such substrates may not be put on spindles and thus "driven" and held in place. Of course, cylindrical substrates having diameters in the toothpick range may not practically be hollow, or spindled, since they would be largely unacceptable for various reasons notably including strength.
In summary, there exists a major need for a practical, economical apparatus which will take solid ceramic cylinders of various sizes, including toothpick diameter, and in a few seconds silk-screen thereon a metered amount of resistive film in numerous desired patterns including noninductive ones. The apparatus must be such as to achieve a great degree of repeatability, with minimized rejects, and to achieve a high degree of surface area utilization. Loading and unloading must be fast and easy, and (very desirably) each machine should be quickly convertible to make different sizes of parts.